The present invention relates to a serial data communication system using a two-write line and more particularly to such a system that a plurality of stations are interconnected by two-wire line comprising a data wire to transfer data therethrough and a clock write to transfer a clock signal for controlling the data transfer.
A serial data communication system of this type is disclosed in European Pat. No. 0051332 published on Apr. 11, 1984. In this system, both the clock and data wires are connected to a power source via the associated pull-up resistor. A master station includes an MOS transistor of an open drain type (or a bipolar transister of an open collector type) having a drain (or a collector) connected to the clock wire, a source (or an emitter) connected to a reference terminal and a gate (or a base) supplied with a clock control signal, while a transmitting station includes a transistor of an open drain (collector) type having a drain (collector) connected to the data wire, a source (emitter) connected to the reference terminal and a gate (base) supplied with a data signal. This communication system, however, has a serious disadvantage that the data transmitting speed is slow, because the clock signal is transmitted to the clock wire by discharging the clock wire by the transistor, and charging the clock wire by the pull-up resistor. The charging time constant is long. Since each bit of the data signal is transmitted in synchronism with the associated clock pulse of the clock signal, the data signal cannot be transmitted at a high speed.